Electron discharge device



v Dec. 3, '1957 v w. w..' WRIGHT ETAL ELECTRON DISCHARGE DEVI-CE Filed Aug. 3. 1953 3 Sheets-Sheet 1 Inventor W-W. WRIGHT- P. WE LCH A.DAY

A itorhey Dec. 3, 1957 w. w. WRIGHT ET AL 2,815,464 ELECTRON DISCHARGE DEVICE Filed Aug. 3. 1953 5 Sheets-Sheet 2 o '56 160.150 260 250 600' Oscillator Grid Current ,uA.

' Inventor N. W WRIGHT F! WELCH' D.A. DAY

A liorney Dec. 3, 1957 W. W. WRIGHT El AL- ELECTRON DISCHARGE DEVICE Filed Aug, 3', 1953 3 Sheets-Sheet 3 Inventor W.W.WR|GHT- P. WELCH- .A-DAY y M United States Patent ELECTRON DISCHARGE DEVICE Walter Wiliiam Wright, Peter Welch, and Dennis Arthur Day, London, England, assignors to International Standard Electric Corporation, New York, N. Y.

Application August 3, 1953, Serial No. 372,(i11

Claims priority, application Great Britain August 3, H52

1 Claim. (Cl. 313-301) The present invention relates to frequency-changer thermionic valves of the heptode type which, as is common in the art, may include within the same envelope a triode local oscillator section to form a triode-heptode valve.

In the hexode frequency-changer valve electrodes are provided whereby the anode current is controlled by a signal grid, supplied from a signal source, and, substantially independently, by an oscillator injection grid supplied from a source of local oscillations, the local oscillator grid being shielded from the signal grid and from the anode by means of two screened grids, commonly joined together internally and in use maintained at a voltage somewhat less than that of the anode of the valve. The anode current-anode voltage curves of the hexode valve for different values of signal and local oscillator voltages are similar in appearance to the family of anode current-anode voltage curves of a tetrode amplifying valve. The heptode valve is similar in construction and function to the hexode except that an additional suppressor grid, as in a pentode, is placed before the anode to suppress secondary emission and to make the anode current substantially independent of anode voltage.

In the conventional frequency-changer circuit a tuned intermediate frequency transformer is inserted in series between the anode of the frequency-changer valve and its high tension supply. In the case, particularly, of a heptode valve using a suppressor grid, in so far as the intermediate frequency circuit is concerned the valve may be considered as a constant current generator supplying a current g times the signal grid voltage, where g is termed the conversion conductance, to the parallel combination of the intermediate frequency circuit and an added impedance Z where Z, is termed the conversion impedance and has a value given by the slope of the anode voltageanode current curve for a given local oscillator grid voltage.

It is evident that desired characteristics of a frequencychanger valve are high conversion conductance coupled with high conversion impedance. It is found, in the case of the heptode with suppressor grid, that although the suppressor grid assists in obtaining a high conversion impedance, it restricts the anode current to such an extent that the conversion conductance is not so high as might otherwise be, while the screen current tends to be large with consequent increase in partition noise. It should be pointed out that the conversion conductance is very dependent upon anode current so that the reduction of anode current by the action of the suppressor grid is a major factor in lowering the conversion conductance.

If, instead of a heptode, a hexode frequency-changer be considered, in which there is no suppressor grid, it is found that if the screen grid nearest the anode be placed closer to the oscillator injection grid than to the anode, the anode current rises at the expense of the screen current. This in itself is desirable, and the conversion conductance tends to be high. On the other hand, if this same screen grid be placed nearer to the anode than to the oscillator injection grid, the anode current falls, the screen current rising, and the conversion impedance is increased at the expense of the conversion conductance.

In the present invention use is made of the above effect by replacing the suppressor grid in a heptode frequencychanger valve by an additional screen grid operated at the same potential as the screen grids to either side of the oscillator injection grid. It should be pointed out that in an embodiment of the invention having similar operating characteristics but improved performance to a given known heptode frequency-changer valve it is not sufiicient merely to replace the connection of the suppressor grid to the cathode with one joining the extra grid to the other two screen grids. 0n the contrary one or more of the grids will have to be re-designed as to the relative positions and pitch of the grid wires to obtain the desired characteristics.

According to one aspect of the present invention, therefore, there is provided a frequency-changer thermionic heptode valve in which the two grids nearest the anode are positioned and dimensioned to function as two screengrids, providing, when similarly polarised, a substantially constant potential between them, the valve thus having hexode characteristics with high conversion conductance and high conversion impedance.

As stated above, if a valve according to the present invention be constructed to be an improvement of an existing design of heptode frequency-changer valve having a suppressor grid, the modifications to the existing heptode design involve a difierent construction as regards dimensioning and positioning of the electrode system. Furthermore, as will be appreciated by those skilled in the art, for a given type of electrode assembly, a given set of opera-ting characteristics leads, in general, to a unique solution for the dimensioning and positioning of the grid wires. The operating characteristics, therefore, may be taken to define the structure. Accordingly the present invention, as viewed from another aspect, provides a frequency-changer thermionic valve comprising a cathode, an anode and a plurality of grids arranged in between the cathode and anode in the following order: a signal grid adjacent the cathode, a first screen grid, a local oscillator injection grid, and a second and a third screen grid, the said grids being positioned and dimensioned so that with the three said screen grids joined together the valve has operating characteristics as hereinafter specified.

In conformity with the usual practice in this class of frequency-changer valves, 2. preferred embodiment of the invention comprises, within the same valve envelope, a heptode section according to the invention together with a conventional triode oscillator section, the triode grid being connected internally to the heptode oscillator injection grid, while the three screen grids may conveniently be connected together internally of the valve envelope.

A frequency-changer valve according to the present invention is especially useful in an otherwise conventional frequency-changer circuit, and in use the invention provides an electri circuit arrangement comprising a signal input and an intermediate frequency output, local oscillator means for generating oscillations to beat with the input signals to produce signals of the said intermediate frequency, and means for mixing the input signal and the oscillations from the said oscillator, the said mixing means comprising a thermionic valve in which the electron stream to the anode is controlled substantially independently of one another by a signal grid adjacent the cathode of the valve and fed from the said signal input and a local oscillator grid fed from the said local oscillator means, the local oscillator grid being shielded from the signal grid and from the anode by means of three screen grids maintained at the same positive potential, the first screen grid being positioned between the signal grid and the local oscillator grid, the second screen grid being placed between the local oscillator grid and the anode, closely adjacent the local oscillator grid, and the third screen grid being placed closely adjacent the anode, the second and third screen grids providing between them a substantially uniform potential for the electron stream.

The invention will be more fully described with reference to the accompanying drawings, in which:

Fig. l is a circuit diagram of a frequency-changer stage of a radio receiver employing a conventional triodeheptode frequency-changer valve.

Fig. 2 shows an equivalent circuit of the intermediate frequency portion of Fig. 1.

Fig. 3 is a schematic representation of a triode-heptode valve according to the present invention.

Fig. 4 shows curves illustrating the performance of valves according to the present invention, and

Figs. 5 and 6 illustrate the construction of an embodiment of the invention.

In a typical conventional frequency-changer circuit, such as shown in Fig. l, a triode-heptode valve 1 may be used having a triode portion V and a heptode portion V The triode-heptode valve represented in Fig. 1 uses an indirectly heated cathode 2 having a centre-tapped heater 3, so that the two halves of the heater winding may be connected in series or in parallel. The cathode is connected to ground through a bias circuit consisting of the resistor R2 shunted by the capacitor C The input circuit comprises an input transformer T1, the secondary winding of which is tuned by a variable capacitor C2, one end of the secondary winding being connected to the signal grid 4 of the heptode V and the other end being grounded through a D. C. blocking capacitor C1. The earth end of the secondary winding of T1 is connected through resistor R7 to a source of automatic gain control bias voltage. The grid 4 is of nonuniform pitch so as to provide a variable-mu characteristic. The grid 5 of the triode section V of the frequency-changer valve is connected to an oscillator transformer T2, the grid winding of which is grounded through a capacitor C8 and is tuned by variable capacitor C7. A grid leak 3 connected between grid 5 and cathode 2 provides bias for the triode section. The anode 6 of the triode section is connected through load resistor R4 and capacitor C9 to the anode winding of transformer T2, the other end of which is grounded. R4 is also connected via resistor R5 to the positive H. T. supply terminal. The grid 5 of the triode section of the frequency-changer valve is connected internally to the oscillator injection grid 7 of the heptode section. Screen grids 8 and 9 are disposed on opposite sides of the oscillator injection grid 7, and are connected together internally. The grids 8 and 9 are maintained at ground A. C. potential by means of capacitor C3, and are connected to the H. T. supply through resistor R1. The anode 10 of the heptode section V is connected through the primary winding of transformer T3 to the H. T. supply. One end of the secondary winding of T3 is connected to ground and the other feeds the intermediate frequency amplifier. These primary and secondary windings are tuned by the capacitors C5 and C6 respectively. The heptode section of the frequency-changer shown in Fig. 1 is completed by a suppressor grid 11 connected internally to the cathode 2. It will be seen that nine external connections are needed to the electrodes of valve 1, including a connection 12 to the centre point of the heater. The valve may thus be mounted upon a conventional 9 pin base.

So far as the intermediate frequency circuit is concerned, an equivalent circuit as shown in Fig. 2 may be drawn in which the transformer T3, tuned by the capacitors 5 and 6, which are now assumed to include the stray circuit capacities together with the anode capacity of the heptode and the input capacity of the I. F. amplifier,

is loaded on the secondary side by the I. F. amplifier input resistance R The primary winding of T3 is shown shunted by Z the conversion impedance of valve V -i. e. the A. C. anode impedance of V at I. F. A constant current generator delivering a current i=g e where e is the signal grid input voltage and g is the conversion conductance of the valve, energises the circuit. It will be seen that for a maximum voltage to be generated across R Z should be small and g should be as large as possible.

A typical radio receiver triode-heptode valve marketed under the code 12AH8 has the following average characteristics when used in a circuit such as Fig. 1 with an H. T. supply of 250 volts:

Heater volts 6.3 or 12.6. Heater current 0.3 or 0.15 amps. Heptode anode current 2.6 ma.

Heptode screen voltage volts. Heptode screen current 4.4 ma.

Signal grid voltage -3 volts. Triode anode resistor (R4) 27,000 ohms. Triode anode voltage 100 volts. Triode anode current 5.7 ma.

Triode grid current 0.2 ma. Oscillator grid volts 0.95 volts (approx-)- Conversion conductance g 0.55 ma./v. Conversion impedance Z 1.5 megohms.

The remaining characteristics which define the electrode structure are the interelectrode capacitances, which for the heptode section are as follows:

Signal input grid to all other electrodes pF 5.0 Heptode anode to all others -pF 8.0 Signal input grid to anode pF 0.025

The above values are obtained when the valve is fitted with a close fitting external shield.

It will be observed that the screen current of the heptode is nearly twice the anode current, so that while the conversion impedance is fairly good, the presence of the suppressor grid tends to make the ratio of screen to anode current rather large and thus reduces the conversion conductance of the tube.

It was mentioned above that in a hexode frequencychanger having no suppressor grid, the conversion conductance and conversion impedance could be varied by altering the position of the screen grid between anode and oscillator injection grid. In an experimental mixer valve otherwise similar to the 12AH8 whose characteristics are given above, the following figures were obtained for two diifcrent positions of this screen grid:

In accordance with the present invention an extra screen grid is used in place of the suppressor grid of the heptode so that between the oscillator injection'grid and the anode there are two screen grids, one close to the oscillator grid and. the other close to the anode. With such a tube, otherwise similar to the 12AH8, the conversion conductance was found to be 865,ua./v. and the conversion impedance 1.55mw. It is seen that with this valve, not only is the conversion conductance improved, but the conversion impedance is also higher.

The arrangement of the electrodes in a triode-heptode embodiment of the invention is represented schematically in Fig. 3, in which electrodes having the same functions as in Fig. l have been given the same reference numerals but the suppressor grid 11 of Fig. l is replaced by the additional screen grid 13, which is shown connected internally to the other two screen grids. This arrangement leaves the external connections the same as in Fig. 1. In use the circuit of Fig. 1 is merely modified by the substitution for the valve 1 of the valve 14 of Fig. 3, the values of certain of the resistors being altered to take account of the changes in electrode currents.

With a 250 volt anode supply and 100 volt screen supply the characteristics of a typical embodiment of the invention are the same as those quoted above for the 12AH8 except for the following:

Signal grid voltage -2.

Heptode anode current 3.8 ma. Heptode screen current 0.95 ma. Conversion impedance 1.55 megohms. Conversion conductance 865,u/a./v.

Allowing for manufacturing tolerances, any embodiments of the invention will have substantially the same dimensions provided that at the same given anode, screen and oscillator injection voltages the various electrode currents and the conversion conductance is within of the figures quoted above, and the conversion impedance is not less than 1 megohm. For the same nominal construction the inter-electrode capacitances should also lie within 20% of those given above. For other embodiments, using possibly cathodes having different heater powers or designed to operate with difierent supply voltages, appropriate scaling factors such as are known to those skilled in the art may be applied to the above quoted figures.

The general effect of the present invention in modifying the conventional triode-heptode characteristics is that the heptode anode voltage-anode current curves become similar to those for a hexode, having a high conversion impedance, i. e. screen grid as opposed to a pentode type of curve is obtained. The essential factors of conversion conductance and conversion impedance vary with oscillator grid current as shown in Fig. 4, in which the full line curves apply to a triode-heptode embodiment of the present invention, otherwise similar to the 12AH8 valve and the dotted curves show the characteristics of the unmodified 12AH8 valve, having a suppressor grid connected to cathode. It is seen that while over part of the range the conversion impedances are much about the same, for oscillator grid currents greater than about ISO/La.- the curves diverge very considerably. The present invention shows a marked improvement over the unmodified valve particularly in respect of the conversion conductance. It will have been noticed that in the embodiment of the present invention the screen current has been very considerably reduced and the anode current increased, which accounts, to a large extent, for the increased conversion conductance. The decrease in screen current also has the effect that the partition noise in the valve is reduced.

In Figs. 5 and 6 there is shown the constructional form of a triode-heptode valve according to the present invention. Fig. 6 is a section through the electrode structure taken along the dotted lines 66 of Fig. 5. The valve is mounted upon a standard novel base 15, the envelope 16 being provided with an exhaust tubulation 17 at the top. The electrode structure is mounted between insulating spacers 18, 19 and 20, with the triode section 21 mounted above the heptode section 22. A central cathode 23 serves both sections, the portion in the triode section of Fig. 5 being obscured by a grid strut 24 of the triode grid 25. The latter is of keyhole shape and effectively beams the electrons to the long sides of the rectangular box-shaped triode-anode 26. A circular anode 27 surrounds the heptode-electrode assembly which comprises a keyhole shaped No. 1 grid 28 (the signal input grid) and four other grids of approximately elliptical shape together with respective pairs of support rods 29.

Grids Nos. 2, 3, 4 and 5 are indicated by the respective reference numerals 30 to 33, grids Nos. 2, 4 and 5 (reference numerals 30, 32, and 33 respectively) being screen grids connected together through straps joining their support rods, and grid No. 3 (reference numerals 31) being the local oscillator injection grid.

In order to illustrate the constructional modifications involved in replacing the suppressor grid of the known heptode valve with the additional screen grid according to the present invention, we list below the essential dimensions of the heptode sections of the 12AH8 valve and the similar triode-heptode valve according to the present invention whose characteristics have been quoted above.

Cathode diameter .045" over coating. Heptode anode diameter .600" internal. Heptode anode length .600".

In the table below the dimension a relates to the embodiment of the present invention and b to the related 12AH8 valve, the dimensions all being in inches.

Major Axis Minor Axis Pitch Wire Diameter Grid No.

a b a b a 1 b 3 a b 120 .059 .059 VA R1 VA R1 .002 .002 210 210 136 136 01565 01565 002 002 310 310 200 020 0182 003 003 410 410 350 350 025 025 003 003 490 490 450 450 040 040 005 005 1 (a) 21 turns at 84TP1, 1 turn at 40'1P1, 21% turns at 84TP1. 2 (b) 19 turns at 76TP1, 1 turn at 36TP1, 19% turns at 76TP1.

It will be seen that grids 1 and 3 are the only ones which have been altered in this case. Rather than to modify grids 4 and 5 in dimensions and position, it was found preferable to use the tube with a higher maximum grid 1 bias voltage and to modify the dimension of grids 1 and 3.

While the principles of the invention have been described above in connection with specific embodiments, and particular modifications thereof, it is to be clearly understood that this description is made only by way of example and not as a limitation on the scope of the invention.

What we claim is:

A frequency-changer thermionic valve comprising a triode section and a heptode section in a common envelope, said triode section comprising a first anode and a grid, a cathode common to said triode section and said heptode section, said heptode section comprising a second anode and five grids only disposed between said second anode and said cathode, in the following order: a signal grid adjacent said cathode, a first screen grid, a local oscillator injection grid connected to the grid of said triode section within said envelope, a second screen grid closely adjacent to said injection grid, said third screen grid immediately adjacent said second anode, an interconnection within said envelope among said screen grids whereby they may all be maintained at a given positive potential with respect to said cathode and whereby said heptode section will exhibit high conversion conductance and high conversion impedance characteristics.

References Cited in the file of this patent UNITED STATES PATENTS 2,125,003 Harries July 26, 1938 2,191,903 Aldous Feb. 27, 1940 70 2,383,345 Seiler Aug. 21, 1945 FOREIGN PATENTS 457,040 Great Britain Nov. 16, 1936 

